PHOENIX (Reuters) – Republican voters in Arizona are set to pick a nominee on Tuesday to replace U.S. Senator Jon Kyl, who is stepping down at the end of the year, with conservative Representative Jeff Flake favored to win.

Flake was leading by a huge margin in opinion polls over Wil Cardon, a feisty political newcomer with deep pockets, in a primary election for a seat that Republicans need if they are to have a chance to take control of the Senate in the November election.

The U.S. Senate contest in Arizona is the highest-profile race among state primaries on Tuesday that include votes in Vermont and Alaska, as well as a runoff election in Oklahoma.

Flake is a six-term member of the House of Representatives, while businessman Cardon oversees his family’s portfolio of real estate investments.

The winner will be the Republican nominee in the race to replace Kyl, a party leader who announced in February he would not seek a fourth six-year term in office.

The Democratic nominee in the November 6 general election is Richard Carmona, a former U.S. Surgeon General and a decorated Vietnam War veteran.

Both Kyl and Arizona’s other U.S. senator, John McCain, have endorsed Flake in a race that started out fiery but has cooled lately.

Cardon came out swinging early with a costly advertising blitz casting Flake as a Washington insider who favored tax increases and was soft on illegal immigration, although the challenger has reduced his spending in recent weeks.

Flake, 49, has offered what he said is a “proven track record” of voting to reduce the size of government and cutting taxes and spending.

Cardon, 41, has contributed nearly $8.8 million of his own money through the August 8 candidate-filing period in a bid to establish himself as an outsider and businessman with experience.

A poll by the Western Representation political action committee released last Friday had Flake leading Cardon by nearly 50 percentage points, or 64 to 16 percent. The survey of 400 Republican voters had a margin of error of 4.9 points.

Republicans already have a majority in the U.S. House and must hold onto open seats such as Arizona’s and gain four in other states to take control of the Senate.

In another hotly contested Arizona primary, two Republican incumbents will battle for the chance to be on the November ballot for a U.S. House seat.

Ben Quayle, son of former Vice President Dan Quayle, takes on David Schweikert, in an expected tight contest brought about by the once-a-decade redistricting. It is one of 11 such House contests between incumbents nationwide this year.